The parallel manipulators are multi-mobile planar or spatial mechanisms with two to six kinematical chains that connect two rigid bodies, generically named platforms. The parallel manipulator is a complex topological structure, having the joints of various types. The most frequently utilized joints are: the mono-mobile kinematical pairs (revolute R or prismatic), the bi-mobile cylindrical pairs C and the tri-mobile spherical pairs S. One of the most important problems in parallel manipulators kinematics is finding the solution of direct kinematics. The paper presents a new way for the calculation of mobility and a new method for the direct kinematics of parallel manipulators RSS type, which are driven by three actuators.